This invention relates to a dryer belt fabric for use in drying paper in the dryer section of a papermaking machine. In particular, the invention relates to a permeable dryer belt fabric of the type which includes a number of plastic helix strips whose winding arcs are joined together by means of a pintle to form a mesh fabric of desired dimension having an open mesh pattern. The helical fabric is normally formed from a thermosetable synthetic resin monofilament. The helical fabric is desirable because it provides a smooth flat surface having fewer knuckles than woven fabric to avoid marking up the paper which contacts the fabric belt while at the same time providing an increased surface area for supporting the paper. The plastic material and belt construction hold up extremely well under the stresses encountered when the belt is travelling endlessly at high speeds, typically 3,000 fpm, about the belt rollers for contacting the paper web being dryed on the dryer machine. Helical dryer fabric is shown in U.S. Pat. No. 4,346,138.
The high permeability of the helical plastic belt fabric provides increased pocket ventilation and drying. However, the problem occurs that the mesh of the helical fabric is considerably open and the permeability of the fabric needs to be reduced in many applications. In particular, for certain grades of paper, the permeability of the helical fabric is too high so that excessive air pumping occurs through the dryer fabric causing the paper web sheet to flutter excessively while being conveyed through the dryer section resulting in marking of the paper or even breaking of the web.
Proposals have been made for reducing the permeability of the helical fabric in paper drying applications. In U.S. Pat. No. 4,381,612 it has been proposed to insert a monofilament stuffer strand through the windings of the individual helix strips and seal the edges of the fabric so that the stuffer strips and helical fabric are made integral. In other embodiments, low-melt nylon strands are inserted in the windings of the helix strips. The strands are sized to insert easily within the windings. Once inserted, subsequent heat treatment causes the nylon strips to melt somewhat and flow to more desirably fill up the openness of the mesh pattern of the material and reduce the permeability. By this means, a helical dryer fabric is provided having good permeability characteristics in certain ranges of dryer fabric permeability. In U.S. Pat. No. 4,362,776 it is proposed to stuff the individual helix strips with a fibrous multi-filament material prior to their being joined together in the fabric to reduce the permeability.
In applicant's co-pending patent application Ser. No. 06/502,255, filed June 8, 1983, entitled LOW PERMEABILITY SPIRAL FABRIC AND METHOD, it is proposed to fill the windings of the individual helix strips with a contoured monofilament stuffing strand which has feathered edges that fit into the corner crevices of the intermeshing winding arcs located in the loop spaces of the windings. By this means, a heretofore open area of the helical mesh pattern that has been unable to be filled is partially closed off to reduce the permeability of the fabric and afford a low permeability characteristic which is desirable for many applications.
In U.S. Pat. No. 4,395,308 a helical fabric is disclosed for a dryer belt for drying paper which includes non-circular pintle strands. By utilizing non-circular cross-sectional monofilaments as pintle yarns to join the individual helix strips together, it is sought to reduce the open area in the mesh and reduce the permeability of the fabric without need of stuffing the windings with filler strands.
The above fabric constructions and methods have provided various ways of reducing dryer fabric permeability for a number of applications. Reducing the permeability of the helical mesh dryer fabric constructions even further is desirable to make their characteristics suitable for very fine grades of paper. Also, the reduction of permeability with decreased labor in the production methods and costs is sought in the development of these fabrics.
Accordingly, an important object of the present invention is to provide a dryer fabric and method for producing a dryer belt having reduced permeability characteristics.
Still another important object of the present invention is to provide a dryer fabric and method by which the permeability of a dryer belt may be reduced for drying paper which are achieved, in part, by using an already carried out step of pintle insertion but utilizing a unique pintle.
Still another important object of the present invention is to provide a dryer fabric and method by which the permeability in the pintle joint area of the dryer fabric is reduced without costly or labor extensive efforts.
Still another important object of the present invention is to provide a dryer fabric and method wherein a composite pintle is used which has a pintle core which forms a pintle joint and a surrounding sheath of a low-melt polymeric material which is caused to soften and deform under heat treatment to fill open areas adjacent the pintle joint.